He said that "a non-exclusive national dialogue with a view to a peaceful resolution of the Chadian conflict" should follow the ceasefire.
The tripartite rebel alliance also wants to see "the installation of a truly democratic political regime" in Chad, which was a French colony prior to 1960, he said.
The Chadian government said its forces had pushed the rebels from the capital after heavy fighting and that the opposition had been "decimated".
"Why a ceasefire? They don't exist any more. With whom would we sign a ceasefire? ... We've got them under control," Nourredine Delwa Kassire Coumakoye, Chad's prime minister, told the France 24 television channel.
An Al Jazeera correspondent in Ndjamena said on Tuesday: "The situation in the city's neighbourhoods is calm. Only government troops are deployed in the streets".
But rebel leaders insisted they had made a strategic withdrawal, and ordered civilians to flee the city of 700,000.
Meanwhile, Nicolas Sarkozy, the French president, said on Tuesday that France could intervene in Chad if necessary.
President Idriss Deby seized power in a Libyan-backed coup in 1990
He went on to win the Chad's first two
multi-party elections in 1996 and 2001
A ceasefire signed between Deby and four rebel groups in October recently collapsed
The largest rebel group, the Union of Forces for Democracy and Development, is led by a former minister who accuses Deby of corruption
Click here for more on Chad's spiral into conflict
"If France has to do its duty, it will do so," Sarkozy told reporters.
"Now there is a legal decision taken unanimously by the Security Council, and if Chad was the victim of an aggression, France could in theory have the means to oppose such action."
In its statement on Monday, the UN Security Council said it "strongly condemns" the rebel offensive and called on UN nations to "provide support ... as requested by the government of Chad".
France has 1,450 troops anda number of Mirage fighter jets stationed in Chad.
Koulamallah has accused French military aircraft of causing "enormous" civilian casualties during the weekend, notably at the Liberte (Freedom) high school and Ndjamena's central market.
In Paris, Christophe Prazuck, a French military spokesman, called the allegation "absolutely baseless".
He said French troops in the capital had only "responded each time they were targeted or caught in cross-fire".
No death toll has been given for the fighting, but many bodies have been seen in the dusty streets, and the aid group Medicins sans Frontieres has told of "hundreds" of civilians wounded.
In Geneva, the UN High Commissioner for Refugees said that 15,000 to 20,000 Chadians have taken refuge in Cameroon to escape fighting between rebels and government forces in Njdamena.
The figure comes from a UNHCR team that has reached the Cameroonian border town of Kousseri, 15km from the Chadian capital, Helene Caux, a spokeswoman for the UN refugee agency in Geneva, said.
"People are still coming through. It's a continuous flow," Caux said, adding that Kousseri was "completely swamped" by refugees.
More than 1,000 foreigners, many of them French nationals, have meanwhile been evacuated from Ndjamena or are awaiting flights out.